Whose Mouth is Cleaner – A Dog’s or a Human’s?

Written by Sara O’Connor

The question about whose mouth is cleaner, a dog’s or a human’s has been around for many years. Some people say you can let a dog lick you all day and it’s perfectly fine. Some will even claim that kissing a person is a worse idea than kissing an animal. Gross!! So, what’s the truth? Well that’s the objective behind performing a science fair project about whose mouth is cleaner. Your experiment and the results will allow you to make a decision and present that decision to the judges. In reality, determining whose mouth is cleaner, a dog’s or a human’s, isn’t the quickest project to put together. But this isn’t too difficult of an idea. In fact, it might even be called a simple process. On the other hand, it will definitely take some time.

You will need to order a few supplies to grow bacteria, mold and other ugly critters. Additionally, there will be a few days of waiting to see what appears. You should probably have around two weeks available to wait for the supplies to arrive and to grow the junk that lives in our mouths. But in the end, if you are careful to do a good job, when the judges look at your science project they will be impressed. Here are the basic steps to properly test whose mouth is cleaner:

  1. Prepare Petri dishes to grow bacteria, mold and other funky stuff.
  2. Swab the mouth of a human with a cotton swab and wipe it on the Petri dish.
  3. Do the same swabbing process for a dog or other animal and apply it to the Petri dish. Don’t get bitten!
  4. Wait for results!

Here’s a more detailed look at the project: Required Items:

  • Water
  • A Container Safe for Boiling Water
  • 6 Petri dishes
  • 6 Cotton Swabs (1 for each Petri dish)
  • Nutrient Agar at least 7 grams (This is a powder that allows bacteria to grow)
  • Plastic Wrap

Need to buy any of these supplies? It shouldn’t cost more than twenty dollars for everything. It is cheapest to use a few cotton swabs you have and buy the agar and dishes separately. These sites are all well-respected and reliable. Just search for “agar” or “petri” dishes once you get there.

  • ScientificsOnline.com.
  • Sargent Welch
  • ScienceKit.com
  • Ward’s Natural Science
  • Steve Spangler Science – Kit including everything needed.

Steps to Prepare the Bacteria Growth:

  1. For each Petri dish you are going to use, probably six, mix about ½ teaspoon of agar with ¼ cup of hot water. So for six Petri dishes, this would be 3 teaspoons of agar, about 7.2 grams, mixed with 1 ½ cups of hot water. WITH ADULT SUPERVISION, bring the mixture to a boil for at least one minute. 
  2. Once the agar mixture has cooled for a few minutes, distribute it to the six Petri dishes equally. Pour it into the both the bottom half of each dish. You will only need to fill them about half way. 
  3. Cover the Petri dishes with your plastic wrap and let the agar cool off. It will slowly harden into a gel. You should probably wait at least an hour for this to take place. 
  4. Now you can prepare your dishes to grow some ugly stuff! You will probably want to take three samples from a human, and three from a dog. But you could also take two from a person, two from a dog and two from a cat, or whatever you please. For each dish, take a cotton swab and wipe it along the inside of the cheek, whether it’s your cheek or you puppy’s. 
  5. Remove the plastic wrap from the Petri dish and then lightly wipe the swab in a squiggly motion all over the surface of the agar in the dish. 
  6. Cover the Petri dish with the lid and label it. You may use a piece of tape to make the label easier to read. It should say something like “Dog Sample #1” so that it is easy to identify later on. Perform steps 4-6 for each Petri dish. 
  7. Now let the creepy stuff grow! Put the dishes in a warm, dark place and wait a few days as the bacteria colonies grow. The temperature shouldn’t be too hot, though or you will risk killing some of the bacteria. 98-100 degrees is probably too hot.

Now what? Well you have your results now. Which Petri dishes seem to have more bacteria and other junk growing in them? Is it the doggy dishes or the human dishes? Count the number of colonies (groups of weird stuff) and make notes about the size and color of the colonies. Make your conclusions and document your results based on what you see. All of this should be done with adult supervision so you don’t get hurt. Additionally, it’s a good idea to keep the Petri dishes closed until you are ready to clean them with a sterilizer like bleach. The bugs growing inside can make you sick if touched or inhaled.

Have fun! Ask for help if needed.

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